LED emitters for generating colored light generally employ the principle of additive color mixing. For this purpose LEDs are normally used that emit the primary colors red, green and blue and can be regulated in their intensity. This method is called RGB method. To improve the color quality, pastel-colored LEDs are also now used for color mixing apart from white or amber LEDs.
The individual color components can be mixed inside or outside the LED emitter. An internal mixing is accomplished, for example, by diffusion by means of diffusing lenses. In the case of external color mixing, collimators or reflectors are used for suitably varying the optical path of the different-colored LEDs, resulting in a superposition of the individual color components on the illuminated object.
Commercially available LED emitters for generating different-colored light are mostly equipped with one reflector only, and are provided with a multitude of different-colored and irregularly arranged LEDs.
A diode lamp with a plurality of different-colored LEDs for readjusting the color temperature is known from DE 600 34 405. A reflecting tube is used for mixing light. The different-colored LEDs are arranged in a matrix in the entrance aperture of the reflecting tube. To mix the individual color components with one another, different geometric shapes of the reflecting tube and different arrangements of the different-colored LEDs within the matrix are suggested.
The main problem of the LED emitters known from the prior art and used for generating different-colored light is that good focusing, high efficiency and good color mixing are incompatible. For instance, systems with internal color mixing by diffusion, for instance by means of diffusers, exhibit poor efficiency and a diffuse and broad emission characteristic. In systems with external color mixing, good focusing properties and high degrees of efficiency are achieved, but color mixing is poor. The diode luminaire known from DE 600 34 405 can be regarded as a compromise. In this instance, too, it is not possible to achieve optimum results in terms of focusing, efficiency and light mixing.
Moreover, the use of modern high-power LEDs necessitates at least passive cooling. As a consequence, when high-power LEDs are used, a standard arrangement of any desired number of irregularly-distributed, closely-spaced, different-colored LEDs in the center of a single reflector, for producing a luminous emitter with acceptable light mixing, is not possible.
WO 2008/010130 A2 shows a composite light source for generating light of a predetermined color. The light source has a plurality of sub-modules which are each able to generate light of that predetermined color. Each sub-module has a light collimating and mixing structure and a light unit group consisting of a plurality of colored LEDs which are arranged at an entrance of the collimating and mixing structure. If, for example, each of four light unit groups comprises four different-colored LEDs arranged in a square array, it is suggested that the LEDs of a sub-module are rotated clockwise or counterclockwise within the array, as compared to a neighboring preceding sub-module, in order to achieve a homogenizing interaction of the light emitted from the sub-modules. Thus, four different arrangements of the LEDs are possible. The rotating pattern is simply repeated if the light source consists of more than four sub-modules.
A disadvantage of the light source described in WO 2008/010130 A2 is that, although better mixing of the light emitted from the sub-modules is achieved by means of the rotated arrangement of the light unit groups, areas of slightly different colored shades can still be observed on a lighted object. This inhomogeneous effect is intensified if the light source comprises more than four sub-modules for a higher light intensity, and the rotating pattern is repeated.